1. Field of the Invention
The invention relates to a power supplying system, more particularly to a load balancer that permits two or more power supplying units of a power supplying system to supply the same amount of power concurrently to operate the same electrical load.
2. Description of the Related Art
Referring to FIG. 1, a conventional power supplying system 1 includes two power supplying units 10, 11 that are connected in parallel to supply power concurrently to operate the same electrical load 12, such as a desktop computer or a notebook computer. However, in the conventional power supplying system 1, the power that is supplied to the electrical load 12 cannot be evenly divided between the power supplying units 10, 11. That is, for a 100-watt electrical load 12, one of the power supplying units 10, 11 might provide a higher power output, such as 80 watts, while the other one of the power supplying units 10, 11 might provide a lower power output, such as 20 watts. Under this condition, the service life of the power supplying unit that provides the higher power output will be shortened, thereby leading to lower reliability. This problem is aggravated when the power supplying system 1 incorporates a larger number of power supplying units.
FIG. 2 illustrates another conventional power supplying system 2 with a load balancing capability. The power supplying system 2 includes two power supplying units 20, 21 that are connected in parallel to supply power concurrently to operate the same electrical load 22. Each of the power supplying units 20, 21 incorporates a resistor (RCS) for detecting the power output of the corresponding one of the power supplying units 20, 21. In the event that one of the power supplying units 20, 21 provides a lower power output than the other one of the power supplying units 20, 21, a load adjusting circuit 23 controls a pulse width modulation (PWM) control circuit 24 that is connected to said one of the power supplying units 20, 21 to increase the power output of the latter until the power outputs of the power supplying units 20, 21 become equal to a balanced power output. For example, assuming that the power supplying units 20, 21 initially provide power outputs of 80 watts and 20 watts, respectively, to a 100-watt electrical load 22, one of the load adjusting circuits 23 will control the corresponding PWM control circuit 24 to increase the power output of the power supplying unit 21 to 50 watts. The power output of the power supplying unit 20 will drop simultaneously to 50 watts at this time.
However, even though the conventional power supplying system 2 incorporates a load balancing capability, it suffers from the following drawbacks:
1. The amplifiers 230 of the load adjusting circuits 23 have an offset voltage limitation. That is, the voltage across input terminals X, Y of the amplifiers 230, i.e. the voltage across the resistor (RCS) of the corresponding power supplying unit 20, 21, must be larger than a predetermined limit in order for the amplifiers 230 to operate. As such, the load balancing capability is only available for large power load conditions. PA1 2. Since the resistor (RCS) is used to detect the output power of the corresponding one of the power supplying units 20, 21, power waste occurs due to the power that is absorbed by the resistor (RCS) whenever the corresponding one of the power supplying units 20, 21 provides a power output to the electrical load 22. For example, assuming that each of the power supplying units 20, 21 provides outputs of +5V/40A, +3.3V/20A, and +12V/16A, and that the resistance of the resistor (RCS) is 5 m , the power that is absorbed by the resistor (RCS) is EQU P=I.sup.2 R=(40+20+16).sup.2 .times.5=28.8W PA1 3. Since the resistor (RCS) is generally formed from a metal material, the resistance thereof is easily affected by ambient temperature. Thus, the power supplying system 2 has relatively poor temperature stability. PA1 4. Since the resistor (RCS) is in use, a voltage drop is present across the resistor (RCS) whenever the corresponding one of the power supplying units 20, 21 provides a power output to the electrical load 22. For example, assuming that each of the power supplying units 20, 21 provides outputs of +5V/40A, +3.3V/20A, and +12V/16A, and that the resistance of the resistor (RCS) is 5 m , the voltage drop across the resistor (RCS) is EQU V=IR=(40+20+16).times.5=0.38 V PA1 5. Good grounding is necessary in the power supplying system 2. Otherwise, severe noise can affect load adjustment of the power supplying units 20, 21, and can result in erroneous operation of the power supplying system 2. PA1 6. In the conventional power supplying system 2, load adjustment is performed by increasing the power output of the power supplying unit 20, 21 that initially provides a lower power output as compared to the other power supplying unit 20, 21. In the event of abnormal operating conditions, the load adjustment as such can easily lead to overvoltage at the output of the power supplying system 2, and can result in damage to internal components of the electrical load 22. PA1 7. Each of the power supplying units 20, 21 is associated with a respective load adjusting circuit 23. Each load adjusting circuit 23 incorporates a relatively large number of discrete components that lead to a relatively complicated construction. This can result in a substantial increase in the cost of the power supplying system 2, especially when the power supplying system 2 incorporates a greater number of power supplying units.